This invention relates to a camera including a solid-state imager which is cooled for reducing temperature-dependent noise and dark current while minimizing power consumption of the cooler.
Portable television cameras having solid-state imagers are now appearing in the marketplace. These cameras are very advantageous, particularly as portable cameras, in that they are extremely rugged, small, consume little power and their imagers tend not to degrade with age. However, they do have certain disadvantages when compared with cameras having conventional pick-up tubes. One disadvantage is the generation of unwanted non-image representative signal components, such as noise and high levels of leakage currents, due to properties unique to solid-state imagers. These properties cause temperature-dependent, and spatially non-uniform black-level leakage currents to appear as signal even in the absence of light, i.e., dark currents. The non-uniformity of the dark currents results in black level shading and/or a color shifting of the subsequently reproduced image. The problem of noise due to temperature-dependent sources and the spatially non-uniform dark currents is exacerbated by the power dissipated in the imager and its support circuitry during operation, which may cause a temperature rise of the imager of perhaps 30.degree. C. above the ambient air temperature outside the camera.
Unwanted temperature-dependent non-image representative signal components, such as noise and dark current, increase with temperature increases. Furthermore, when operating the camera in low illumination scenes, the amount of light reaching the imager may be reduced from a preferred level and result in a reduced level of image-representative signal being supplied from the imager. Thus, although a camera including solid-state imagers may provide good noise performance and black level shading when operating at room temperature under ordinary lighting conditions, the noise performance and black level shading may not be satisfactory under low light level conditions, such as may be experienced when televising night scenes or indoor scenes under ambient lighting. Consequently, it is desirable to cool a solid-state imager to reduce the level of temperature-dependent noise and dark currents in its output signal when the camera is operated in scenes having low illumination, so that satisfactory signal-to-noise ratio (S/N) and low black level shading is obtained for the imager output signal. The cooling may be accomplished by a thermoelectric cooler, which may be a solid-state device through which current is passed in order to pump thermal energy away from a cooled surface in accordance with the well-known Peltier phenomena. However, some of the advantages of the solid-state imager over the tube-type imager are lost when cooling is used e.g., the additional power required for the thermoelectric cooler increases the power drain on the power supply for the camera. Since a majority of solid-state imager cameras are portable and operated from batteries, the camera operating time between battery recharges is undesirably reduced due to the increased power drain of the cooler. If the thermoelectric cooler is constantly used to maintain the solid-state imager at a predetermined temperature, unnecessary power is utilized when the camera is operated under conditions wherein satisfactory signal-to-noise performance could be obtained with less or no cooling. It is therefore desirable to minimize the power drain caused by a thermoelectric cooler without noticeably sacrificing the S/N or black level uniformity performance of the camera.